Lattice-mismatched GaAs Research Articles

Investigation of bulk and surface minority carrier lifetimes in metamorphic InAsSb grown on GaAs and Si

Monolithic integration of III–V-based optoelectronic devices onto Si wafers provides enormous benefits to many device manufacturing technologies. Therefore, it is essential to understand the effect of limiting factors such as dislocations on the material properties. In this paper, we study the minority carrier lifetimes in mid-wave infrared InAsSb alloys grown on lattice-matched GaSb and lattice-mismatched semi-insulating GaAs and Ge/Si substrates. Time-resolved microwave reflection measurement has been performed to study the carrier dynamics and different recombination mechanisms over the temperature range of 20–300 K at various optical injection levels. The sample on GaAs is found to have a lower lifetime over the entire temperature region than the sample on the Ge/Si substrate. The threading dislocation density values estimated from the lifetime analysis were found to be 2.9±0.2 times larger in the sample on GaAs when averaged over the temperature range of 70–200 K. Furthermore, we studied the contribution of various recombination mechanisms, and it was shown that the lifetime in the sample on GaAs is dominated by the Shockley–Read–Hall recombination up to 140 K, above which the Auger recombination is the limiting factor. This transition temperature is 80 K and 100 K for the samples on GaSb and Ge/Si, respectively. We have also investigated the effect of the surface recombination on the total lifetime. The extracted bulk lifetime was found to be up to 2× higher when the surface effect was excluded.

Photoluminescence spectroscopy of metamorphic InAsSb on GaAs and Si

Improving the production cost and yield in next-generation infrared focal plane arrays have drawn attention towards the growth of absorbing material on large-area low-cost substrates while maintaining high material quality. In this paper, we study the structural and optical properties of InAsSb bulk layers grown on lattice-mismatched semi-insulating GaAs and Ge-on-Si (Ge/Si) substrates using molecular beam epitaxy. We used X-ray diffraction (XRD) and temperature-dependent photoluminescence (PL) spectroscopy to determine the overall quality of these samples w. r.t the grown sample on the lattice-matched GaSb substrate. Analysis of the PL spectra's line shape reveals the ground state bandgap energy along with various optical transitions in each sample in the temperature range of 12 K–280 K. Furthermore, the temperature dependence of PL peak energy was analyzed, and the Integrated PL (IPL) intensity as a function of inverse temperature was used to compare the PL efficiency in each sample. The results indicate that PL is less dependent on the lattice mismatch-induced dislocations while it is mainly affected by the cracks originated from the difference in the thermal expansion coefficient of the Si substrate and the epilayer. The combination of optical and imaging analysis presented in this paper provides a deeper insight into each sample's material quality and predicts a promising path toward the application of alternative substrates in FPA imageries.

Influence of GaAs and GaSb substrates on detection parameters of InAs/GaSb superlattice-based mid-infrared interband cascade photodetectors.

The paper presents electrical and optical properties of interband cascade infrared photodetectors with InAs/GaSb type-II superlattice absorbers. We compare the detection parameters of detectors grown on the native GaSb substrate and lattice-mismatched GaAs substrate and seek solutions to enhance device performance, specifically with using an optical immersion. The detectors grown on GaAs have better detection parameters at room temperature, but, at lower temperatures, the misfit dislocations become more important, and detectors grown on GaSb become better.

Molecular beam epitaxy growth and characterization of interband cascade infrared detectors on GaAs substrates

Abstract Interband cascade infrared detectors (IB CIDs) are devices built from multiple cells connected in series using specially designed interband tunneling and relaxation regions. Such design enables effective collection of photogenerated carriers and is particularly beneficial in the case of short diffusion length in absorber’s material. In this work, we report on the growth and characterization of type-II InAs/GaSb superlattices IB CIDs on highly lattice-mismatched (0 0 1) GaAs substrates for mid-wave range. IB CIDs are characterized by high resolution X-ray diffraction, dark current and current responsivity. The performance of devices with different number of stages is discussed. Devices with 50% cut-off wavelength at 5.3 μm exhibit at temperature 300 K peak detectivity of 3.6 × 108 cmHz1/2W−1.

Excitons in InGaAs quantum dots without electron wetting layer states

The Stranski–Krastanov growth-mode facilitates the self-assembly of quantum dots (QDs) by using lattice-mismatched semiconductors, for instance, InAs and GaAs. These QDs are excellent photon emitters: the optical decay of QD-excitons creates high-quality single-photons, which can be used for quantum communication. One significant drawback of the Stranski–Krastanov mode is the wetting layer. It results in a continuum close in energy to the confined states of the QD. The wetting-layer-states lead to scattering and dephasing of QD-excitons. Here, we report a slight modification to the Stranski–Krastanov growth-protocol of InAs on GaAs, which results in a radical change of the QD-properties. We demonstrate that the new QDs have no wetting-layer-continuum for electrons. They can host highly charged excitons where up to six electrons occupy the same QD. In addition, single QDs grown with this protocol exhibit optical linewidths matching those of the very best QDs making them an attractive alternative to conventional InGaAs QDs.

Low temperature growth and extrinsic doping of mono-crystalline and polycrystalline II-VI solar cells by MBE

An investigation of low temperature MBE growth of crystalline Cd(1-x)Zn(x)Te with 0 ≤x ≤ 1, in the range 220 ℃≤Ts ≤ 320 ℃, and extrinsic doping is conducted. This results in the construction of homo-junction crystalline solar cells on lattice-mismatched 211B and 111B GaAs substrates. Doping and composition studies of crystalline layers are used as a guide for low-temperature, high growth-rate, MBE grown polycrystalline CdTe solar cells on various n-type emitters and different back-contacts. The best resulting devices reach efficiencies of 14.7% under AM 1.5 g, without an anti-reflection coating.

Mid-infrared metamorphic interband cascade photodetectors on GaAs substrates

Antimony-based mid-infrared interband cascade (IC) photodetectors fabricated on (001) GaAs substrates are reported. By using a “buffer-free” interfacial misfit array growth method, an overall good crystalline quality is obtained on the largely lattice-mismatched GaAs substrate. The GaAs-based IC detectors show comparable optical performance, with similar electrical performance at temperatures higher than 140 K, as compared to the reference devices grown on GaSb substrate. The GaAs-based IC detectors demonstrate dark current density of 2.63 × 10−6 A/cm2 at 180 K, which is about twice as compared to that grown on GaSb substrate, with Johnson-limited D* of 1.06 × 1011 Jones at 180 K and 4.0 μm. The results indicate that IC detector design is robust and relatively insensitive to the material quality, and metamorphic IC detector is viable for large-format infrared focal plane array applications.

(Invited) P-Type III-Sb MOSFET on a Metamorphic Substrate: Towards All III-V CMOS

Group III-Sb materials have the best hole transport among all the III-V semiconductors and therefore are promising candidates for future p-type MOSFETs for all-III-V CMOS ICs. Progress is reported on development of strained InGaSb p-type channels grown by molecular beam epitaxy with metamorphic buffers on lattice-mismatched GaAs and Si substrates with the emphasis on specific defects, surfaces and interfaces and electrical characteristics of these materials, structures and devices. Optimization of MBE growth on GaAs (001) substrates has resulted in “step-flow” growth mode of GaSb with monolayer-high steps on the surface, about 107cm-2 dislocation density and bulk hole mobility of 860 cm2/Vs. Strain optimization in InGaSb quantum wells (QW) has provided the highest Hall mobility of 1020 cm2/Vs at sheet hole density of 1.3x1012/cm2 obtained for In0.36Ga0.64Sb with compressive strain of 1.8%. Hole mobility in buried QW channel with Al2O3 high-k gate oxide has shown just a minor 30% drop of the mobility due to interface-related scattering to ~700 cm2/V-s in the surface QW channels. An InAs top capping layer reduces the interface scattering even further; the sample with 3nm total top barrier thickness demonstrates mobility of 980 cm2/V-s and shows sheet resistance of 4.3 kΩ/sq., very close to the minimum QW resistance in the bulk. Larger lattice mismatch and non-polar nature of silicon present a further challenge for growth of high quality III-Sb materials. Formation of defects is analyzed and baseline for surface morphology and defect densities (threading dislocations, microtwins and antiphase boundaries) are presented. Defects formed on Si substrates produce significantly higher background p-type doping and stronger scattering than that on GaAs substrates. A similarly strained InGaSb QW with InAs capping layer and 3nm total top barrier thickness has shown the highest mobility of 630 cm2/V-s on a Si substrate. Both n- and p-type MOS capacitors were studied with various thicknesses of III-Sb buffers and consequently, with different defect densities. Be-doped p-type GaSb MOS capacitors demonstrated similar characteristics to MOSCaps grown on GaAs substrates. In contrast, we observed a p-type MOSCap behavior in GaSb:Te on Si devices despite Te doping of up to 5x1017 cm-3 in the structures with thin, <1.5mm III-Sb structures. Thicker structures with dislocation densities in the top layers <108 cm-2 have shown normal n-type C-V behavior similar to the structures grown on GaSb and GaAs substrates, but with significantly shorter minority carriers generation/recombination lifetimes. SIMS analysis did not reveal noticeable Si diffusion from the substrate, nor other impurities were detected in GaSb. Specific morphological features on the surface, were further analyzed using TEM, FIB/SEM and AFM. Two major types of surface topography defects were found: shorter crystallographically aligned straight steps resulting from microtwins and creating the surface steps with the height equal to the number of faulted planes; and longer winding loops corresponding to antiphase domain boundaries confirmed by direct STEM imaging of the high/low-z Sb/Ga dumbbell contrast. Cross-sectional TEM reveals the density ~6x108 cm-2 of threading dislocations within top layer of epi structure with total thickness of 1.25 um, and ~2x104 cm-1of planar defects. Based on the finding of low thermal budget (<4500C) of the III-Sb/oxide interfaces, two novel technology paths have been developed for MOSFFETs on GaAs substrates: gate-first self-aligned with in-situ MBE gate oxide; and gate-last with InAs etch-stop layer, epi-contacts and ALD oxide combined with optimized dry and wet etching techniques to improve the interface and reduce the access resistance. The resultant MOSFETs have shown close to record-high drain currents.

Local electrical properties of n-AlInAs/i-GaInAs electron channel structures characterized by theprobe-electron-beam-induced current technique

We developed a probe-electron-beam-induced current (probe-EBIC) technique to investigate the electrical properties of n-Al(0.48)In(0.52)As/i-Ga(0.30)In(0.70)As electron channel structures for a high-electron-mobility transistor, grown on a lattice-matched InP substrate and lattice-mismatched GaAs (001) and Si (001) substrates. EBIC imaging of planar surfaces at low magnifications revealed misfit dislocations originating from the AlInAs-graded buffer layer. The cross-sections of GaInAs channel structures on an InP substrate were studied by high-magnification EBIC imaging as well as cathodoluminescence (CL) spectroscopy. EBIC imaging showed that the structure is nearly defect-free and the carrier depletion zone extends from the channel toward the i-AlInAs buffer layer.

Determination of the thickness and spectral dependence of the refractive index of Al x In1 − x Sb epitaxial layers from reflectance spectra

A nondestructive method for measuring the thicknesses of epitaxial layers of AlxIn1 − xSb alloys based on interference effects in reflectance spectra measured in a wide wavelength range (1–28 μm) is implemented. The studied 0.9–3.3 μm thick AlxIn1 − xSb layers are grown on highly lattice-mismatched GaAs substrates by molecular beam epitaxy. The found thicknesses are in good agreement with the independent data of scanning electron microscopy. The spectral dependence of the refractive index n(E) of AlxIn1 − xSb layers is measured both for the regions of transparency and fundamental absorption. The refractive index for the case of E E0 are found based on the interference pattern.

Features of molecular-beam epitaxy and structural properties of AlInSb-based heterostructures

AlInSb layers are grown on highly lattice-mismatched GaAs (100) substrates by molecular-beam epitaxy (MBE) and studied in situ by reflection high-energy electron diffraction and ex situ by scanning and transmission electron microscopy (SEM and TEM). It is shown that one feature of AlInSb/GaAs heterostructure features is a high probability of forming microtwins; methods for decreasing their concentration are proposed. To initiate AlInSb growth on GaAs substrates under high lattice-mismatch (∼14.5%) conditions and to stimulate the transition to 2D growth, the GaAs layer surface was preliminarily exposed to an antimony flux followed by deposition of an intermediate AlSb buffer layer. The optimization of initial MBE growth stages of Sb-containing layers on the GaAs surface allows a decrease in the defect density in the GaAs/AlInSb heterostructures more than by two orders of magnitude, including a drastic decrease in the microtwin density. Optimal MBE growth conditions for AlxAl1 − xSb are determined in a wide composition range (0 < x < 0.3). The TEM and SEM studies confirm the high structural quality of grown GaAs/AlInSb heterostructures. Hall-effect measurements showed the dependence of the carrier mobility and concentration on the aluminum content in AlInSb layers and allowed preliminary conclusions on scattering mechanisms.

Capture kinetics at deep-level defects in MBE-grown CdTe layers

The results of deep-level transient spectroscopy (DLTS) investigations in n-type CdTe layers grown by the molecular-beam epitaxy (MBE) technique on lattice-mismatched GaAs substrates are described. Three electron traps and one hole trap, at rather low concentrations of the order of 1013 cm−3, have been revealed in the DLTS spectra measured under various bias conditions of Schottky diodes prepared on the as-grown CdTe layers. One of the electron traps has been attributed to electron states of dislocations on the ground of the logarithmic capture kinetics for capture of electrons into the trap states. The other three traps, displaying exponential capture kinetics, have been attributed to native point defects produced during the epitaxial growth of CdTe. The microscopic nature of the defects responsible for the traps is discussed taking into account recent results of first-principles calculations of the properties of dominant native defects in CdTe.

Intense Terahertz Radiation from InAs Thin Films

Terahertz (THz) radiation from InAs thin films grown by molecular-beam epitaxy on closely lattice-matched p-type GaSb (100) substrates and lattice-mismatched semi-insulating GaAs (100) substrates was investigated. The THz radiation intensity was measured from InAs films with thicknesses between 100 nm and 1.5 μm excited by a femtosecond laser pulse with a wavelength of approximately 780 nm. The radiation intensity increased as the InAs film thickness increased and it exceeded that from a bulk n-type InAs substrate with an electron concentration of 2.3 × 1016 cm−3 when the InAs film thickness was greater than about 500 nm. In addition, the THz intensity from a 1-μm-thick InAs film was greater than that from a bulk p-type InAs substrate. We ascribe this enhanced THz intensity to the wave reflected from the lower interface between the InAs film and the layer grown beneath it. We confirmed this by observing an increased pulse width due to constructive overlap of the reflected wave. The results demonstrate that InAs thin films are promising materials for THz emitting devices.

Properties of InSbN grown on GaAs by radio frequency nitrogen plasma-assisted molecular beam epitaxy

We report the growth of InSbN on a lattice-mismatched GaAs substrate using radio frequency nitrogen plasma-assisted molecular beam epitaxy. The effects of a two-step thin InSb buffer layer grown at 330 and 380 °C and substrate temperature (270–380 °C) on the properties of the InSbN are studied. The crystalline quality of the InSbN is significantly improved by the two-step buffer layer due to defect suppression. The shifting in the absorption edge of the InSbN from ∼5 to 8 µm following an increase in the substrate temperature is correlated with the reduction in free carrier concentration from ∼1018 to 1016 cm−3 and increase in concentration of N substituting Sb from ∼0.2 to 1%. These results will be beneficial to those working on the pseudo-monolithic integration of InSbN detectors on a GaAs platform.

Comparison of short period InAs/GaSb superlattices on GaSb and GaAs substrates

Type II superlattices (SLs) short period InAs(4ML)/GaSb(8ML) were grown by molecular-beam epitaxy on lattice-mismatched GaAs substrates and on GaSb substrates. A smooth GaSb epilayer was formed on GaAs substrates by inserting mulit-buffer layers including an interfacial misfit mode AlSb quantum dot layer and AlSb/GaSb superlattices smooth layer. SLs grown on GaAs substrates (GaAs-based SLs) showed well-resolved satellite peaks in XRD. GaSb-based SLs with better structural quality and smoother surface showed strong photoluminescence at 2.55 μm with a full width at half maximum (FWHM) of 20 meV, narrower than 31 meV of GaAs-based SLs. Inferior optical absorption of GaAs-based SL was observed in the range of 2–3 μm. Photoresponse of GaSb-based SLs showed the cut-off wavelength at 2.6 μm.

Growth mechanisms and crystallographic structure of InP nanowires on lattice-mismatched substrates

We present a growth model that predicts the growth phase and mechanism of InP nanowires (NWs) and the experimental verifications of the model. The NWs were grown on lattice-mismatched GaAs substrates using metal-organic chemical vapor deposition via Au nanodrop-assisted vapor-liquid-solid growth. Nanodrops with larger diameters are shown to grow longer NWs because growth is governed mainly by direct precursor impingement on the nanodrop surface. The theoretical and experimental results also show that growth phase is dependent on NW diameter. We show that InP NWs with a diameter less than a certain value exhibit coherent growth of a single crystalline wurtzite (WZ) phase, whereas larger diameter InP NWs often contain sequences of WZ and zincblende phases and stacking faults. These findings allow one to achieve coherent NW growth and WZ phases free from twinning if the NW diameter is below certain material-dependent critical diameters.

Molecular beam epitaxy growth of InAs and In[sub 0.8]Ga[sub 0.2]As channel materials on GaAs substrate for metal oxide semiconductor field effect transistor applications

InAs and high indium concentration InGaAs have very high electron mobilities and saturation velocities. Using them as the metal oxide semiconductor field effect transistor (MOSFET) channel materials is a very promising way to keep improving the integrated circuit chip performance beyond Moore’s law. One major obstacle is the growth of these high mobility channel materials on lattice-mismatched substratcs. In this work, we studied the molecular beam epitaxy growth of InAs, In0.8Al0.2As, and In0.8Ga0.2As on lattice-mismatched GaAs substrate using a thin indium-rich InAs wetting layer. Reflection high energy electron diffraction and atomic force microscopy were used to optimize the growth conditions. A surface roughness of ∼0.5nm rms was obtained for InAs layers. A new MOSFET structure with In0.8Ga0.2As channel and In0.8Al0.2As buffer layer was also demonstrated. High mobility depletion mode MOSFET characteristics were demonstrated.

Self-assembled lateral InAs quantum dot molecules: Dot ensemble control and polarization-dependent photoluminescence

By proper control of underlying templates, lateral InAs quantum dot molecules (QDMs) can be grown with a desired number of quantum dots (QDs) per molecule. We demonstrate that by capping as-grown InAs QDs with lattice-mismatched GaAs layer and by varying the capping temperature between 430 and 470 °C, we are able to control the span of nanoholes template to between 150 and 300 nm. InAs regrowth on this template result in nanopropellers (0.6 ML regrowth) or QDMs (1.2 ML), the latter with 4–12 QDs per molecule depending on the span of the underlying nanoholes template. All QDM samples grown share a characteristic feature with a clear preference to orient along the [ 1 1 ¯ 0 ] crystallographic direction. Room-temperature photoluminescence measurements show a strong polarization-dependent characteristic, in good agreements with the QDM’s geometry.

Surface-phase equilibria in MBE of strained In1-xGaxAs heteroepitaxcial layers

A thermodynamic analysis of surface-phase equilibria in the MBE growth of In1-xGaxAs layers on lattice-mismatched GaAs and InP substrates is done. It is shown that, in In-stabilized conditions, the substrate-induced strain enhances the arsenic equilibrium pressure over In-Ga-As liquid phase in the whole range of the solid phase composition in the case of GaAs substrate, whereas in the case of the InP substrate, the pressure is enhanced by the strain at x<0.47 and lowered at x>0.47. For both substrates, locking the surface monolayer composition occurs. In the As-stabilized conditions, the strain results in the lattice-pulling effect. [DOI: 10.1380/ejssnt.2006.53]

High-mobility InSb epitaxial films grown on a GaAs (001) substrate using liquid-phase epitaxy

The growth of epitaxial InSb layers on highly lattice-mismatched semi-insulating GaAs substrates has been achieved via traditional liquid-phase epitaxy. Scanning and transmission electron microscopy show sharp interfaces even at 35 nm resolution. High-resolution x-ray diffraction studies reveal reflections even up to 2θ=153° with distinct layer and substrate peaks, indicating structural coherence. The films grown were n type and the highest electron mobility obtained was 3.96×104 cm2/V s at room temperature. The band gap varies from 0.17 to 0.23 eV in the temperature range of 300–10 K and is consistent with the expected variation. These results indicate that the films grown are comparable to those grown by other sophisticated techniques in terms of structural, optical and electrical properties.